This invention relates to circularly polarized antennas and, more particularly, to such antennas which include a conductive cylinder having a slot for purposes of achieving horizontally polarized radiation.
In the recent past, there have been significant developments that have led to circularly polarized broadcasting, which has been found to improve television reception in large metropolitan areas. However, up until the present time, circular polarization has been exploited only in connection with VHF television broadcasting because current innovative antenna designing has occurred in, and been limited to, that region of the spectrum.
One of the advantages of circular polarization has been the elimination of reception problem areas because almost any desired pattern can be achieved, either omni-directional or selectively directional depending on the desired area of coverage. Further advantages are a reduction in ghosting, better isolation between closely channeled antennas, and a more solid reception coverage since the reception is independent of the receiving antenna's orientation.
At the present stage of development of antennas, however, circular polarization has not been extended to the UHF channels.
Accordingly, a primary object of the present invention is to achieve the extension of circular polarization in an efficient and relatively inexpensive manner to the UHF channels as well as VHF.
It is well known that a slot cut in a waveguide wall, whether the guide be coaxial, rectangular, or circular, will radiate energy into space in the plane perpendicular to that of the slot's long dimension. In broadcast antenna applications, the slots are positioned vertically on a cylindrical pylon antenna to emanate a horizontally polarized signal. An example of a slotted antenna producing a horizontally polarized signal is that described by Bazen in U.S. Pat. No. 2,981,987.
Accordingly, another primary object of the present invention is to provide a means of converting the type of antenna exemplified by the Bazen patent so as to enable circularly polarized radiation.
Attempts have been made to convert slotted antennas of the type disclosed in Bazen, and an example of a circularly polarized antenna of this type may be appreciated by reference to U.S. Pat. No. 4,129,871 to Mckinley R. Johns.
In the Johns patent, a circularly polarized antenna is provided using a slotted conductive mast. The slots extend about one half wavelength long along the lengthwise axis of the mast for exciting horizontal components of the wave. A pair of conductive rods extend from respective affixation points closely adjacent each elongated side of an individual slot, each rod being about one full wavelength long and having a free end portion extending in the vertical plane approximately one half wavelength to radiate the vertical component of the wave.
As noted by the patentee in U.S. Pat. No. 4,129,871, each of the conductive rods disclosed therein is considered an end fed, full wave radiator. This contrasts with the dipole of the antenna system of the present invention which, rather than being end fed, parasitically develops the required polarized energy, such dipole--or at least a portion thereof--being in the same horizontal plane as the polarized energy emanating from the slot formed in the cylindrical mast of the system. Therefore, the dipole incorporated in the antenna system of the present invention is termed a parasitic dipole because of the manner of its coupling to the horizontally polarized energy.
It is another object of the present invention to maintain a good, i.e., constant axial ratio throughout the elevation pattern of radiation from the antenna.
Previously known designs of slot driven antenna systems, such as those involving interlaced slots and dipole elements, have led to unacceptably high axial ratios with increased depression angle. Infinite axial ratios and left-handed polarization can result. Consequently the location of in-home "rabbit ear" antennas becomes critical since positions can be found where the received signal goes to zero. In contrast, the design in accordance with the present invention ensures that phase quadrature and good axial ratio will be maintained with increased depression angle.